Leaky Seattle dam causes flood worries

Posted: Tuesday, September 29, 2009

KENT, Wash. - For nearly 25 years, Kathy Gladden has lived about 100 feet from the Green River, a normally placid stream that meanders past the many homes, offices, warehouses and shopping malls that blanket the area.

Now, she and thousands of others face the all-too-real prospect that the river will gush past a leaky upstream dam and swallow up their homes once the rainy season starts in November, devastating a heavily developed area in the Seattle suburbs that is a vital hub of commerce.

The Howard Hanson Dam began showing disturbing signs of vulnerability after a torrential rainstorm in January, and officials have been warning residents to buy flood insurance, stow valuables in safe places and be ready to flee.

King County has declared a state of emergency that allows it to seek federal reimbursement and speed up work to bolster levees, while the Army Corps of Engineers bought 400,000 sandbags and other flood-fighting materials in the round-the-clock fight to save the dam.

"I can't bear it. It's awful," said Gladden, 72, who hates the idea of having to leave her mobile home park and its tight-knit community. "I never even heard of the Howard Hanson Dam until the trouble started."

The dam, located in the Cascade foothills east of Seattle, has prevented major floods in the Green River valley since it was completed in 1962. That changed when last winter's heavy rains weakened a hillside next to the dam.

A record 15 inches of rain fell in 12 hours on the Green River's upper watershed in January, sending torrents into the 235-foot-high dam's reservoir. The reservoir rapidly filled 6 feet higher than ever before.

The dam held the water back and remained sound. But at the high water levels, engineers saw worrying signs within the dam's right abutment, a 450-foot-wide pile of rock deposited by a huge landslide 10,000 years ago.

As a temporary fix, the corps is spending $8.9 million to inject grout into the abutment, forming a shield to lessen the seepage. Without such work, Col. Anthony Wright, the corps' Seattle District commander, said there would be a 1 in 3 chance this winter for flooding in the Green River Valley.